Environmental remediation approaches by nanoscale zero valent iron (nZVI) based on its reductivity: a review

The fast rise of organic and metallic pollution has brought significant risks to human health and the ecological environment. Consequently, the remediation of wastewater is in extremely urgent demand and has received increasing attention. Nanoscale zero valent iron (nZVI) possesses a high specific surface area and distinctive reactive interfaces, which offer plentiful active sites for the reduction, oxidation, and adsorption of contaminants. Given these abundant functionalities of nZVI, it has undergone significant and extensive studies on environmental remediation, linking to various mechanisms, such as reduction, oxidation, surface complexation, and coprecipitation, which have shown great promise for application in wastewater treatment. Among these functionalities of nZVI, reductivity is particularly important and widely adopted in dehalogenation, and reduction of nitrate, nitro compounds, and metal ions. The following review comprises a short survey of the most recent reports on the applications of nZVI based on its reductivity. It contains five sections, an introduction to the theme, chemical reduction applications, electrolysis-assisted reduction applications, bacterium-assisted reduction applications, and conclusions about the reported research with perspectives for future developments. Review and elaboration of the recent reductivity-dependent applications of nZVI may not only facilitate the development of more effective and sustainable nZVI materials and the protocols for comprehensive utilization of nZVI, but may also promote the exploration of innovative remediation approaches based on its reductivity.


Introduction
With the growth of population, the development of modern industry, and the accelerating economy, the demand for clean water resources is gradually increasing, 1,2 while these valuable water resources have been seriously threatened by the growing intractable and unmanageable water pollution, originating from the discharge of industrial, domestic, and agricultural effluents. 3,4Wastewater usually contains various containments, including dyes, halogenated compounds, pesticides, antibiotics, metal/metalloid ions, radionuclides, nitrates, and so on, which can bring signicant risk to human health and the ecological environment via bioaccumulation, antibiotic resistance, carcinogenesis, and ecotoxicological effects. 5,6][9] In recent decades, nanoscale zero valent iron (nZVI) and its composites have been employed for wastewater remediation, and performed signicant removal capacity of various pollutants, indicating a high potential in practical application. 10,11ZVI was rst synthesized via the reduction of Fe(II)/Fe(III) with borohydride in 1995, and adopted in the dechlorination of chlorinated organics in 1997. 12From then onwards, the preparation and application of nZVI have been extensively studied. 13,14As the fourth most prevalent element and the second most abundant metallic element on the earth, iron is abundant, environmentally friendly and useful for application in environmental remediation. 15,16Compared with zero valent iron (Fe 0 ), which is a reactive metal with standard redox potential (E 0 = −0.44V), nZVI has a smaller size (1-100 nm), higher specic surface area, and thus greater reactivity. 17The core-shell structure and Fe-containing oxide layer endow nZVI with distinctive reactive interfaces, which provide plentiful active sites for the reduction, oxidation, and adsorption of contaminants. 18,19Given these abundant functionalities of nZVI, it has undergone signicant extensive studies on environmental remediations, linking to various mechanisms, such as reduction, oxidation, surface complexation, precipitation, which have shown great promise for the treatment of wastewater.
0][21] During the reduction or aqueous storage process, the oxidation of Fe 0 surface layer by H 2 O and O 2 can lead to the formation of a unique reactive oxide lm on the surface of nZVI, which provides abundant active sites for surface complexation and adsorption of contaminants and their subsequent transformation via oxidation or reduction pathways. 16,224][25] Concomitantly, more remediation approaches, such as adsorption-reduction, and adsorption-reduction-precipitation in variably oxic or anoxic environments, have emerged based on the reductivity of nZVI. 19,26Moreover, nZVI can be easily recycled with an external magnet, and the recycling is still viable even if the nZVI has undergone surface oxidation or surface passivation. 18,27This character endows nZVI with environmental benignancy and convenient recyclability in the applications of remediation or enrichment, necessitating it as a promising candidate for environmental applications.
Accordingly, nZVI has attracted great attention and has been widely applied in wastewater remediation based on its reductivity via various approaches, such as chemical reduction applications, electrolysis-assisted reduction applications, and bacterium-assisted reduction applications.However, there is a lack of papers that critically analyse the recent advances in the applications of nZVI in environmental remediation based on its reductivity, especially in the comprehensive examination of the multiple progressive environmental remediation approaches based on the reductivity of nZVI.In this regard, this review aims to provide an overview of the recent advances in the reductivitydependent environmental remediation approaches by nZVI (Scheme 1), and to gain insight into the current progress, deciencies, and future improvement in the reduction application of nZVI.
As nZVI has a large specic surface area and high reactivity, it is prone to be corroded by H 2 O and dissolved O 2 in solutions, leading to dissolution and surface passivation.The oxidation of nZVI with H 2 O can produce H 2 , which is vital for the reduction of contaminants. 41While surface passivation of nZVI can result in deterioration of reductivity, which is a primary obstacle in the reduction application of nZVI. 77Moreover, dispersed nZVI particles are susceptible to agglomerate and form chain-like aggregates as a result of magnetism and van der Waals forces, thus giving rise to a decrease in specic surface area and reductive reactivity. 78,79Thereby, several strategies have been employed in laboratory research and eld trials to overcome these obstacles, such as modifying and emulsifying nZVI, doping nZVI with catalytic noble metal (i.e., Pd, Ni, Cu), and supported nZVI onto solid porous materials. 12,80The welldesigned functionalized support gives nZVI material with enhanced adsorption capacity and reductivity, triggering a cooperative remediation strategy of adsorption-reduction.In the reductive and adsorption-dechlorination applications, nZVI showed substantially high reactivity and removal efficiency towards organic chlorides. 81,82Moreover, supported nZVI materials not only have good adsorption performance towards metal/metalloid ions, but also can reduce them to low valent metal ions or metallic state, hence realizing the removal and enrichment of metal/metalloid ions from aqueous solution via magnetic separation. 83,841.Dechlorination Chlorinated organics are usually toxic, mutagenic, and carcinogenic to human beings and the ecosystem.85 Due to their extensive use and refractory nature, the presence of these compounds in water bodies is almost ubiquitous.86 Efficient dechlorination of chlorinated organics by nZVI is highly depend on its reductivity. Ina typical dechlorination process, H 2 O is rstly reduced and generates H 2 (eqn (1)), which is subsequently activated by the doped catalytic metal (i.e., Pd) and forms activated hydrogen atom (H$), executing hydrodechlorination towards contaminants.Accordingly, nZVI with high reductivity can produce H 2 at a higher rate, and perform superior dechlorination efficiency.
2.1.1.Direct dechlorination.Direct dechlorination of chlorinated organics via hydro-dechlorination has been widely studied, in which Cl atom is replaced with H atom and forms much less toxic hydrocarbons.The application of nZVI in the dechlorination of TCE has been comprehensively studied, and the long-term application of nZVI in the actual water systems has been a research hotspot in recent years. 87 ). 29his study reveals that different water chemical condition leads to different anaerobic dissolution processes of surface layer and corrosion processes of nZVI (dissolution corrosion), and result in different dechlorination processes of TCE.Specically, the dissolution corrosion of Ca 2+ -HCO 3 − towards nZVI gives rise to the exposure of Fe 0 from the passivation layer, which promotes the reductive dechlorination of TCE (Fig. 1).While Ca 2+ -SO 4 2− and Na + -NO 3 − have a relatively slight dissolution corrosion effect on nZVI (Fig. 1).In future practical remediation applications of nZVI in contaminated sites, taking the effect of water chemical conditions on nZVI corrosion and dechlorination performance into consideration may be necessary.
Regarding that the cleavage of aromatic C-Cl bonds is more difficult than that of aliphatic C-Cl bonds driven by the electron transfer process by nZVI, efficient dechlorination of chlorinated aromatic compounds commonly calls for the doping nZVI with Pd or Ni, which provides hydrogenation activity for dechlorination by catalytic transforming H 2 to H$ (Fig. 2A). 28,88,89A comparative study conducted by Venkateshaiah et al. found that the order of dechlorination efficiency by bimetallic nZVI is: nZVI/Pd > nZVI/Ni > nZVI/Ag > nZVI/Cu (Fig. 2B). 28Zhuang et al. prepared graphitized carbon supported nZVI/Ni, which performed satised dechlorination efficiency on TCP compared with pristine nZVI, owing to the generation of atomic H$ and the inhibition of oxidation and deactivation of nZVI by graphitized carbon. 40Liu et al. employed nZVI/Ni in the dechlorination of TCE, the efficiency of which exhibited 1.4-3.5 times higher than that of nZVI. 31A study conducted by Anang et al. focused on the transformation and the fate of attapulgite supported nZVI/Ni during the dechlorination, which is vital for the predicting of nZVI surface chemistry, providing guidance for more utilization nZVI, and studying the impact of the corrosion process on environment. 34Their research indicates that the Fe 0 core is rstly hollowed, collapsed, and then gradually formed poorly crystallized Fe 5 O 3 (OH) 9 at the rst dechlorination stage, which later transformed to g-FeOOH, a-FeOOH, and Fe 3 O 4 by the end of the dechlorination (Fig. 3).The doping of Ni and the supporting on attapulgite can signicantly accelerate the dechlorination and this transformation.
Besides doping, some scholars use enhancement techniques or external articial intervention measures to improve the effectiveness of nZVI in the dechlorination of chlorinated aromatic compounds.Blundell et al. applied ultrasonic energy in the reductive dechlorination of DDT, which prevents the aggregation and surface deactivation of nZVI, leading to a signicant increase in the dechlorination efficiency of DDT. 33hi et al. capsulated nZVI into Fe 3 O 4 @SiO 2 to stimulate magnetic spatial connement effect and change the electron transfer pattern, which realized a controlled interfacial electron transfer behavior and the prevention of formation oxide surface layer on Fe 0 , achieving an increased dechlorination efficiency on TCP of 5.53 times in magnetic eld compared with that of pristine nZVI without magnetic eld.Despite that H-transfer is a well-recognized dechlorination mechanism, e-transfer between nZVI and target contaminant has also been proposed as a complementary mechanism that may occur in dechlorination without a catalyst.Brumovský et al. prepared nitrogen-doped nZVI (Fe x N) by passing a gaseous NH 3 / N 2 mixture over nZVI at elevated temperatures to enhance the dechlorination performance of nZVI by changing the hydrodechlorination mechanism to direct electron transfer mechanism (Fig. 4). 30Fe x N showed a 20-fold increase in the TCE dechlorination rate compared with pristine nZVI and retained high reactivity even aer three months of aging.While, its dechlorination performance on more stable chlorinated aromatic compounds is still lack of knowledge.
2.1.2.Adsorption-dechlorination. When supported nZVI onto porous support, coupling the adsorption function of support with the reductive dechlorination of nZVI, provides a novel strategy for dechlorination, which enhances the interfacial dechlorination reaction and improves dechlorination efficiency.Shang et al. supported nZVI/Pd in HF or NaOH-modied biochar (FBC-nZVI-Pd, SBC-nZVI-Pd), and demonstrated a signicantly enhanced removal efficiency of 98.8% in 48 h toward TCB, attributing to the active sites providing by pore-lling spaces of HF-modied biochar (FBC) or NaOH-modied biochar (SBC) for the adsorption of Fe 2+ during the preparation of nZVI and the adsorption of TCB during dechlorination (Fig. 5). 39The SBC-nZVI-Pd has increased specic surface area, aromaticity, and hydrophobicity aer the modication, endowing it with excellent adsorption performance   towards organic pollutants (Fig. 5).The study indicated that pollutants are rstly enriched and subsequently dechlorinated.In the rst stage (12 h), the proportion of dechlorination is about twice that of adsorption, while the proportion of adsorption is gradually increased as the reaction reaches an equilibrium in the residual stage.Zhang et al. modied spent bleaching earth carbon with cetyltrimethylammonium bromide and loaded with nZVI, for the dechlorination of DCF. 44This work indicated that the synergistic strong adsorption effect of the material and reduction effect of nZVI lead to a highefficiency removal of DCF (87%).Huang adopted reduced graphene oxide supported nZVI/Ni for the rapid adsorption (p-p interaction between 2,4-DCP and graphene nanosheet) and dechlorination 2,4-DCP, achieving a removal efficiency of 95% in 3 h. 36The composites had good stability and high recycling value, which can be repeatedly used ve times without obvious decrease in efficiency (Fig. 6A).The dechlorination always prefers appropriate acidity conditions, which could accelerate the corrosion of nZVI and produce sufficient hydrogen for the subsequent formation of reactive atomic hydrogen by Ni, thus facilitating hydro-dechlorination (Fig. 6B).While, excessive acidic conditions may lead to a fast loss of nZVI/Ni and form excessive hydrogen bubbles at the interface of nZVI/Ni, resulting in diminished dechlorination efficiency on the contrary (Fig. 6B).
Normally, the support is not only adsorbent for pollutants, but also benecial for the distribution of nZVI and prevention of the aggregation of nZVI owing to the connement and dispersion effect. 90Meanwhile, some scholars reported that biochar can also enhance the conductivity of bimetallic nZVI/Ni, thus promoting the production of dominant reactive atomic hydrogen, and demonstrating remarkable TCE degradation efficiencies of 91.8% in tap water. 21A strong adsorption effect may not always benecial for the dechlorination.Xu et al. discovered that the adsorption state of PCB1 on support has a prominent effect on the dechlorination by nZVI/Pd and nZVI/ Ni (Fig. 7). 42,43As PCB1 is difficult desorption from black carbon from low pyrolysis temperatures, dechlorination is inhibited, the efficiency of which is only 53.5% in 48 h. 42While, the electron releasing of Fe 0 and the generation of H$ is enhanced in high temperature black carbon due to its high conductivity, thus reducing the inhibition of adsorption of PCB1 on dechlorination to some extent, achieving an efficiency of 95.3% in 48 h.Their ndings suggested that characteristics of support and adsorption state of pollutants may heavily affect the dechlorination efficiency of nZVI.
In addition, polystyrene resin, bentonite, Al-based MOFs (MIL-96), and zinc-based zeolitic imidazolate framework (ZIF-8) have also been employed in the embedding of nZVI.Zhang et al. indicated that 2,4-DCP is completely dechlorinated by polystyrene resin supported nZVI/Ni, and the generated phenol can be adsorbed and magnetic separation from water. 35As reductive dechlorination prefers acid conditions, the presence of CO 3 2− had a signicant inhibition effect on dechlorination (Fig. 6C).Baldermann et al. immobilized nZVI on bentonite substrate, forming "micro-reactors" for the sorption of TCE on the clay surface and the subsequent dechlorination. 91Bentonite prevents the agglomeration or inactivation of nZVI due to the 9% within 120 min owing to the synergism of adsorption and reduction. 37Benetting from the activity protection of nZVI/Pd by ZIF-8, the removal efficiency maintained relatively high at 73.7% even aer 5 repeated uses (Fig. 6D).

Other dehalogenation
2.2.1.Debromination.Brominated ame retardants, which have been widely used as additives in circuit boards and plastics to improve ame resistance, are of signicant concern to human health owing to their endocrine disruptive risks and immune toxic characteristics. 48,49Debromination is vital for the treatment and disposal of organic bromide.
Chen et al. reported the ball milling of nonmetallic particles from waste-printed circuit boards with nZVI, the results of which indicated that the content of bromine on the surface of the nonmetallic particles was reduced by 50%. 92The debromination is not only attributed to the electron transferred from nZVI during ball milling, but also the reduced energy required to break the C-Br bond promoted by the stretch and length increase of the C-Br bond aer pentabromodiphenyl ether gained electrons from nZVI.Huang et al. added nZVI in vacuum low-temperature pyrolysis of resin particles from waste-printed circuit boards and achieved 69% debromination efficiency, which was obviously higher than that without nZVI (20%). 47The existence of nZVI changes the degradation pathway of organic bromides by providing electrons and H, transforming organic bromine of resin into xed inorganic bromine (Fig. 8).In the debromination of BDE-47 without nZVI, the C-O bond may break rstly and the C-Br bonds will break in sequence, and eventually formed phenol and benzene via the reaction of two monomers with hydrogen radicals generated from pyrolysis (as shown in path 1.).While, the presence of nZVI in the debromination of BDE-47 will make the C-Br bond more likely to fracture by providing electrons, resulting in a change of bond breaking sequence, that is, the C-Br bonds may break earlier than C-O bond (as shown in path 2.).The following pathway for the debromination of BDE-25 depends on the bond breaking sequence of C-O bond and C-Br bonds in para-position.If the C-Br bonds in para-position are more likely to break than C-O bond, the debromination reaction will continues, otherwise subsequent debromination will occur in two monomers as shown in path 1. Notably, ∼83% of nZVI can be separated and reused in the debromination.
The debromination efficiency by nZVI alone is usually low.To achieve higher efficiency and more complete debromination of organic bromides, Pd, Cu, and Ni are oen coated on nZVI surface.Compared with nZVI, the debromination efficiency by Pd-doped nZVI was about 2-4 orders of magnitude greater in the debromination of mono-, di-, and tri-brominated diphenyl ethers. 93Li et al. employed Cu-nZVI in the batch debromination of TBBPA (10 mg L −1 ), which was mostly transformed to bisphenol A within 240 min (Fig. 9). 48While, without doped Cu, there was almost no obvious debromination occurred (Fig. 9C).As low pH is benecial for the formation of H$ and inhabitation of the deactivation of nZVI, the debromination efficiency by Cu-nZVI reached maximum value at pH 5 (Fig. 9D).Huang et al. further indicated that catalytic debromination is dominated in the removal of TBBPA in the condition of pH 3-7, high temperature ($25 °C), and Cu loading; while, adsorption will be dominated in the condition of alkaline condition (pH $ 11), low Cu doping, or low temperature (#15 °C). 49Moreover, in the recyclability tests of Cu-nZVI for the debromination of TBBPA, the removal efficiency remained roughly high throughout the experimental cycles, suggesting no obvious substantial drop in catalytic performance (Fig. 9E).
As regard to the mechanism of the accelerated dehalogenation by doped nZVI, scholars present H-transfer and e-transfer processes.In the H-transfer mechanism, the doping metal with high standard redox potential acts as a catalyst to improve the dehalogenation, via (i) the forming of galvanic cells with nZVI, enhancing Fe 0 corrosion and H 2 formation; (ii) catalysing hydrogenation by promoting the transformation of adsorbed H 2 to H$. 48In the e-transfer mechanism, the doping metal forms a galvanic couple with nZVI, enhancing the electron transfer (etransfer) from nZVI to contaminants, thus enhancing the dehalogenation. 46 10A). 46The possible debromination pathways of BDE-47 in various bimetallic systems are summarized in Fig. 10B.In the e-transfer process, orthobromine of BDE-47 is preferentially debrominated and generate BDE-28, whereas in the H-transfer process, para-bromine of BDE-47 is preferentially debrominated and generate BDE-17.The debromination reactivity of these bimetallic systems at 1% (w/w) metal additive loading follows as: Fe/Pd > Fe/Ag > Fe/ Cu > Fe/Ni > Fe/Au > Fe/Pt z nZVI (Fig. 10C and D).
2.2.2.Deuorination.As the chemical reactivity of C-F is signicantly lower than C-Cl and C-Br, research on deuorination by nZVI is insufficient. 94Huang et al. used activated carbon-supported nZVI (Fig. 11A) to remove orfenicol (FF),  which is a widely used halogenated antibiotic. 95The removal of FF was demonstrated as adsorption and dehalogenation, the former of which is dominated by van der Waals forces, hydrogen bonding, and chemisorption (e.g., p-p interaction and Fe-O bond).The adsorbed FF was subsequently suffered from dechlorination and deuorination by nZVI-AC, and generated no harmful products (Fig. 11B and C).The removal of FF performed a pH and Cr(VI) concentration-dependent response (Fig. 11D and E), and Cd(II) slightly affected the removal efficiency (Fig. 11F).Acidic conditions would facilitate the formation of H 2 and prevent the passivation of nZVI by providing abundant H + ions, and thus promote the removal of FF.The redox reaction of Cr(VI) with nZVI and Fe(II) may enhance the electron transfer and hence promote the removal of FF by nZVI.However, Cr(VI) may compete with FF for reactive sites in the condition of high concentration of Cr(VI).As regard to the coexisting Cd(II), it could not only increase the ionic strength and restrain the iron oxide precipitation and enhance the FF removal, but also could compete with FF for reactive sites and inhibit the removal of FF, resulting in a slight impact on the FF removal on the whole.While humic acid has obvious suppression on the removal of FF (Fig. 11G), due to the competitive adsorption of humic acid (HA) on the shells of nZVI by chelating.

Reduction of metal/metalloid ions, nitrate, and nitro
Metal/metalloid ions, such as Cr(VI), Pb(II), and Sb(V), which are widely used in many elds, have become common toxic pollutants due to improper storage or uncontrolled emission. 56hey can exist in the environment for a long time and get into human bodies via inhalation and contact, posing an increasing risk of disease and cancer for human beings. 56,62The extensive use of chemical fertilizers and nitrate-containing materials has accelerated the excessive accumulation of nitrate in water bodies, resulting in hydration and red tide in eutrophicated water. 76,96Nitro compounds, which are widely utilized in the production of insecticides, explosives, and dyes, have become one of the major organic pollutants in water owing to their degradation-resistant characteristics, carcinogenicity, and teratogenicity on organisms. 54n recent years, extensive research efforts have been dedicated to endowing nZVI materials with multiple functions and expanding the applications of nZVI towards these pollutants, leading to the development of various nZVI-based composites that not only preserve their reductive potential but also outperform their counterparts in the recovery of pollutants from environment.
2.3.1.Direct reduction.Highly toxic Cr(VI) can be reduced to less toxic Cr(III) by nZVI (eqn (2) and ( 3)).Hao et al. prepared nZVI by a low-cost green synthetic method using plant extracts for the reduction of Cr(VI). 97These biomolecules can cap on the Fe 0 surface and prevent the oxidation and inactivation of nZVI.Similarly, Cheng et al. adopted CaCO 3 coated on nZVI for the reduction of Cr(VI), which performed long-term stability. 98Their study demonstrated that the presence of Ca 2+ and Mg 2+ facilitated the reduction of Cr(VI), but the PO 4 3+ and HA impeded the reduction.Although nitrate reduction by nZVI-based materials has been extensively studied, the aggregation and surface passivation of nZVI, interference of coexisting ions, and the preference of unfavored ammonia products limit the application of this technology.Huang et al. supported nZVI/Pd on graphene to overcome the aggregation of nZVI and the extensive formation of ammonia, in which the N 2 selectivity was enhanced from 0.4% to 15.6% and the nitrate removal efficiency was 97%. 99olymers, including polyacrylamide (PAA), carboxymethyl cellulose (CMC), polyethylene sorbitan monolaurate (PSM) and polyvinylpyrrolidone (PVP) have also been employed for the inhibition the aggregation of nZVI during the reduction of nitrate, in which the reduction efficiency followed the order as 99.5% (PVP-nZVI) > 99% (PAA-nZVI) > 97% (PSM-nZVI) > 70% (CMC-nZVI) > 55.6% (pristine nZVI). 75Moreover, scholars discovered that doped metal (Ni, Ag, Cu) can promote electron transfer and minimize oxidation of nZVI, and alkaline conditions are favorable for the removal of nitrate. 74Some optimization experiments indicated that the main effects that impact the reduction efficiency of nitrate followed the order as: aging time > pH > temperature. 76itro compounds can also be primarily reduced to amino compounds, which are more prone to be bio-degraded. 54Gu et al. employed graphene/biochar in the supporting of nZVI to enhance the electron-releasing capacity and the electron transfer from nZVI or Fe(II) to -NO 2 , achieving a removal efficiency of 77.1% towards nitrobenzene. 54Deng et al. demonstrated a high dose of Ag on nZVI will not only prevent the aggregation of nZVI, but also facilitate the nZVI peeling, thus benet for the reduction of p-nitrophenol; while, low dose of Ag on nZVI has an adverse effect (Fig. 12). 52ased on its high reductivity, investigations of reductive recovery of Cu and Ni from polluted rivers, reductive degradation of organophosphate esters, and reductive degradation of sulfamethoxazole by nZVI have also been reported. 45,68.3.2.Reduction-precipitation.In the reduction of Cr(VI) by nZVI, the generated Cr(III) can form co-precipitate with Fe(II) and Fe(III) (eqn (4)), which can signicantly alleviate the bioavailability of Cr in water of soil.57 Rice straw-derived hydrothermal carbon and cellulose lter paper have been used as support for nZVI, aiming to improve the dispersibility and oxidation resistance of nZVI during the reduction of Cr(VI).100,101 Most of the Cr can be recovered from water utilizing the magnetically separating of nZVI material.While the cover of precipitate may reduce the activity of nZVI.Therefore, Liu et al. employed nZVI supported by xanthan gum-modied reduced graphene oxide for the remediation of Cr(VI)-polluted aquifer, in which Cr(VI) is reduced to Cr(III) by Fe 0 and then adsorbed by the negatively charged oxygen-containing functional groups on rGO and formed precipitate attached to rGO, rather than on the surface of nZVI.56 While, Hu et al. conned PAA on the surface of nZVI by Al(OH) 3 , the results of which showed that the surface carboxylic groups of PAA bound generated Cr(III) and Fe(III) and suppressed the precipitation of hydroxides on the surface of nZVI (Fig. 13A).58 Thus, Cr(VI) reduction capacity was improved from 49.4 to 92.6 mg g −1 within 24 h.A study conducted by Liu et al. conrmed that alkaline conditions are unfavorable for the reduction of Cr(VI) (Fig. 13B) due to the rapid precipitation of iron ions on the surface of nZVI, preventing the reaction of Fe 0 with Cr(VI).57 In addition, Ca 2+ , Mg 2+ , SO 2.3.3.Adsorption-reduction. Adsorption is identied as the most reliable method for the enrichment of pollutants with low concentration or high mobility, which will promote the following remediation process undoubtedly.
Li et al. developed a surface phosphate modication for nZVI to enhance the adsorption of Cr(VI) by revising the monodentate mononuclear model on pristine nZVI into bidentate binuclear one on the phosphate nZVI surface, thus promoting a removal efficiency of Cr(VI) by 4 folds. 60Using ammonium thiocyanate functionalized graphene oxide supported nZVI, Wang et al. found that the adsorption process follows as pseudo-secondorder model and Langmuir-Hinshelwood rst-order model at the reduction process, and the removal of Cr(VI) was dominated by chemical surface-limiting step. 103In the study conducted by Shu et al., they identied that the adsorption-reduction of Cr(VI) by almond shell-derived biochar/nZVI prefers acidic conditions rather than alkaline conditions (eqn ( 5) and ( 6)). 104The results indicated that N-H was the main functional group responsible for the chemisorption process, and the removal efficiency of Cr(VI) was 99.8% at pH 2-6, which was approximately 20% higher than that at pH 7-11.Other functional groups, i.e.R-COOH, R-OH, R-NH 2 , and R-C-O-C on nZVI materials could also provide active sites during the chemisorption process, which can be described by Sips adsorption isotherm model and pseudo-second order model as well, involving adsorption, surface complexation, reduction and ion exchange reaction (Fig. 14).(Fig. 15A and B). 63The adsorption of positively charged metal ions is promoted by surface electronegativity deriving by HPO 4 , which is formed through a typical dehydration process of dihydrogen phosphate groups (Fig. 15C).Meanwhile, the radial nanotracks (CnZVI), generated from the ensile hoop stress on nZVI by phosphorylation, promote the facile inward diffusion of Ni(II) and the rapid outward transfer of electron and Fe(II) through oxide layer, namely galvanic replacement (Fig. 15D-G).
Han et al. discovered that boric acid and borates can be transformed as B-B/B-Fe on nZVI, providing highly electron-decient Lewis's acid sites for the effective gathering of NO 3 − and OH − , leading to a high-efficiency adsorption-reduction of nitrate in a wide range of pH (5-9). 73Yang et al. entrapped nZVI in the matrix of cyclodextrin polymer for the adsorption and  reduction of p-nitrophenol, and the material had a long-term stability of 109 d. 51 Surfactants, such as rhamnolipid and sodium dodecyl sulfate have also been applied in the strengthening of the adsorption and reduction of nitrobenzene by nZVI from soil. 53dsorption and decontamination of hydrophobic BDE-3 from aqueous solutions is commonly difficult.Sophorolipid-modied nZVI, which possessed more accessible active sites and reduced charge transfer resistance compared to pristine nZVI, enhanced the solubilization of BDE-3 via halogen bonding interaction, giving signicant facilitation on the adsorption and subsequent reduction by nZVI and achieving a removal efficiency of 99.96%. 55hen the composition of contaminants is complex, competitive adsorption or reaction may occur.Cai et al. iden-tied that Pb(II) and Ni(II) will compete the limited effective adsorption/reaction sites in binary mixtures, and Pb(II) owns greater competitive ability than Ni(II). 65Liu et al. modied nZVI/ rGO with xanthan gum to fabricate a stable reaction zone, for the breaking of the negative synergistic phenomenon between NO 3 − and Cr(VI). 1053.4.Adsorption-reduction-precipitation.The efficient removal of non-biodegradable metals/metalloids has been one of the top priorities in wastewater remediation, thus making the development of corresponding green technologies of great signicance.63 In recent years, extensive research efforts have been dedicated to enhancing the removal of metal/metalloid ions by the method of adsorption-reduction-precipitation, leading to the development of novel nZVI magnetically separable composites that make the thorough removal of metals/ metalloids or recovery of resources from wastewater achievable.Continuous and inexhaustible efforts have been devoted to the exploration of nZVI material with high and selective adsorption capacity, high and sustained reductivity, and high separability.
Converting highly toxic Cr(VI) to low-toxic and separable immobilized Cr(III) by nZVI via adsorption-reduction-precipitation has garnered increasing research attention.As there are various limitations of pristine nZVI during production and application, including easy agglomeration, fast oxidation, and easy deactivation, 61,106 nZVI is commonly loaded in porous materials, such as bentonite, 107 attapulgite, 108 SBA-15, 109 graphene, 110 biochar, [111][112][113][114][115][116] resin, 117 hydrogel, 118-120 membrane, 121,122 MOF 106 etc.The supporting not only prevents aggregation and deactivation of nZVI, but also endows nZVI materials with high and/or selective adsorption towards Cr(VI).Xu et al. implemented a series of spectroscopic investigations and veried that various oxygen-bearing functional groups on biochar benet the adsorption of Cr(VI) via complexation and electrostatic interaction. 113The protonation of -NH 2 and -OH on support can also signicantly enhance the adsorption of Cr(VI) via electrostatic attraction, leading to a dominant chemisorption mechanism following a pseudo-second order model. 108,116In addition to the contribution to adsorption, carbon and GO can also provide good electrical conductivity and long-term electron-releasing properties, accelerating the reduction of Cr(VI) to Cr(III). 110Moreover, the reactivity of supported nZVI can be further improved by modifying supported nZVI with stabilizers, such as soluble starch, polydopamine, and glutathione. 108,111,121,123Besides adsorption and reactivity of nZVI, the removal efficiency of Cr(VI) is also subjected to material status, interfere ions, pH, and catalyst.Normally, nZVI materials in suspension (without drying process) performed higher removal efficiency compared with powder ones, acidic and anaerobic conditions facilitate the reaction. 123,124Acetate ions could promote the Cr(VI) removal, but HA, nitrate, and carbonate ions resulted in negative effects, while SO 4 2− and Cl − have slight effect on the removal efficiency. 61,109The removal of Cr(VI) can be hugely enhanced by the catalyzing of Pd or Ni, which catalyze the formation of H$ and benet for the rapid reduction of Cr(VI) (eqn ( 7) and ( 8)), leading to a complete reduction of Cr(VI) in 5 min. 109,125All in all, the removal of Cr(VI) by nZVI involved three steps, (i) adsorption of Cr(VI) by active sites, (ii) reduction of Cr(VI), and (iii) formation of Fe-Cr-O precipitates; the adsorption for ∼30% on the removal of Cr(VI), the reduction/ precipitation accounts for the rest ∼70%. 108,118,125Lei et al. reported a simultaneous adsorption-reduction-precipitation removal of Cr(VI) and 4-NP by polypyrrole-supported Pd/nZVI (Pd/Fe@PPY), which achieved complete removal of Cr(VI) and 4-CP within 1 min and 60 min respectively (Fig. 16A-C). 126The presence of Cr(VI) signicantly impaired the dechlorination of 4-CP attributing to the deposition of Cr(III)-Fe(III) co-precipitates on nZVI, while the presence of 4-CP had a minor effect on the removal of Cr(VI) and total Cr, which is more obvious in recycle experiments (Fig. 16D).Similarly, the combined removal of Cr(VI) and 4-CP prefers acid conditions rather than alkaline conditions (Fig. 16E).
U(VI) and U(IV) are two dominating states of Uranium, the former of which is higher in mobility and solubility in aquatic environments, lower bioavailability, and more chemical toxicity. 70,127Therefore, the removal and transformation of U(VI) from contaminated environment has been an urgent matter to be solved.Scholars conrmed that the reduction of U(VI) by nZVI could easily form precipitates and thus eliminate their immobilization in the natural environment (eqn ( 9) and ( 10)).Zhang et al. employed nZVI loaded chitosan in the synergistic adsorption (Langmuir isotherm adsorption) and reduction of U(VI), achieving a high removal quantity of 591.72 mg g −1 . 70The well-dispersed nZVI owed high reactivity and is considered as electron donor in the chemical reduction of mobile U(VI) to precipitated U(IV) on nZVI.Li et al. developed an adsorptionreduction-solidication strategy for the elimination of U(VI) by MCM zeolite-supported nZVI, yielding a U(VI) removal capacity on nZVI/MCM of 216 mg g −1 . 127The surface functional groups, including -OH, Fe-O, and Si-O, are benecial for the adsorption of U(VI).Hua et al. explored the technique of reduction, enrichment, and separation of U from U-tailings wastewater in a continuous-ow device, decreasing the concentration of U from 331 mg L −1 to 1.47 mg L −1 in the continuously dealing of ∼500 L radioactive wastewater in 193 h. 72iu et al. presented a novel collaborative strategy for enhancing the removal of U(VI)/Cr(VI) by MBenes/SBA-15 supported nZVI, in which U(VI)/Cr(VI) were rapidly and abundantly adsorbed via electrostatic interaction and reduced by surfaceassociated nZVI to U(IV)/Cr(III), and nally, the generated U(IV)/ Cr(III)/Fe(III) rapidly formed co-precipitation on the nZVI composite's surface (Fig. 16F-J). 23,71The competitive adsorption of U(VI) and Cr(VI) can occur in an insufficient adsorption sites situation, which may result in low removal efficiency. 71 The adsorption-reduction-precipitation method has also been applied in the recovery of As(V), Ni(II), and Pb(II).Fan et al. adopted nZVI/biochar in the reduction of As(V) to As(III), which was subsequently absorbed by amorphous FeOOH and coprecipitated on nZVI. 69Sang et al. demonstrated the formation of NiO, Ni(OH) 2 , and Ni during the recovery of Ni(II) by rhamnolipids modied nZVI, suggesting a reduction-adsorption-precipitation mechanism involving in ref. 66.Similarly, precipitate PbO, Pb(OH) 2 , Pd 3 (CO 3 ) 2 (OH) 2 , and Pb 0 can be discovered on nZVI during the remediation process, revealing the synergistic effects of adsorption, reduction, and precipitation. 67

Concept for future
When employing nZVI in the reduction of contaminants, its reducing activity and long-term stability are signicant for the reaching of its full potential in practical application.Although nZVI demonstrated superior reduction of numerous organic contaminants, metal ions, and nitrate from wastewater, weak van der Waals forces and intrinsic magnetic interactions led to strong homo-aggregation of nZVI, posing unavoidable challenges to its reactivity and long-term performance.Meanwhile, nZVI with high reducing activity is vulnerable to faster dissolution and surface passivation, also giving drawbacks to its long-term stability.Besides, nZVI with low reducing activity is incompetent in providing sufficient reductivity for contaminants.Therefore, high-activity cultivation and maintenance technologies and surface passivation protection technologies, which can prevent the inactivation of surface-active sites of nZVI and sustain its high activity during application, may be a necessity.The present solution is loading nZVI in/on porous materials to disperse nZVI and prevent their aggregation, alleviate their oxidation and deactivation, and further endow nZVI materials with higher adsorption capacity or cooperative remediation effects.Future efforts may focus on the further improvement of reductivity and long-term stability of nZVI to deal with refractory contaminants and broaden their reductive application, as well as the electrochemical properties of support.Such as expanding the application of nZVI in dehalogenation, i.e. debromination and deuorination, to deal with the increasingly severe organic uorine and organic bromine pollutions. 128,129uture studies may also develop nZVI materials with high/ specic affinity towards contaminants by regulating the surcial characteristics, devoting to the remediation of actual wastewater with low concentration.Considering that the components of actual wastewater are always complex, investigations and deeper view on the interactions and mechanisms during the remediation process also need to be claried.
As for the comprehensive utilization of nZVI, the dissolving Fe(II) during dechlorination may have a secondary utilization value, as a study has shown that Pd/Fe(OH) 2 has reductive dechlorination on TCE. 32Thus, utilization protocols of dissolved Fe(II) generated from the corrosion of nZVI and the mechanisms involved in are deserve to be explored.

Electrolysis-assisted reduction by nZVI
Recently, an electro-remediation technology using ZVI has been proposed for the remediation of metal ions, organic pollutants, and nitrate.With the additional electrons provided by the electric current, the reduction of contaminants by nZVI can be enhanced. 130i et al. proposed an efficient technique to enhance the removal of Se(IV) from wastewater via the electrolysis-assisted reduction by nZVI (E-nZVI), which achieved an enhanced removal efficiency (15.83 mg Se(IV)/g nZVI) exceeded nZVI system by ∼135%. 131Separate anode-cathode experiments and surfacesensitive quantitative characterization demonstrated that improved performance is not only attributed to the synergistic effects of nZVI and cathodic reduction on the precipitation of Se, but also benet from the nZVI corrosion aggravated by electrochemical oxidization in the anode chamber (Fig. 17).Kinetic tests indicated that lowering the resistance (elevating electrolyte concentration) and raising the applied voltage (4.0 V or more) can give rise to promotion in the removal efficiency (Fig. 17), but cannot give rise to apparent promotion on the reaction rate constant.
Pavelková et al. conducted dechlorination tests of chlorinated hydrocarbons (TCE, PCE, and DCE) by nZVI with the electric eld and indicated that the dechlorination predominantly occurred around the anode despite that it is expected near the cathode. 130No dechlorination products was detected in the cathode owing to the presence of Fe oxides, such as Fe 3+ or Fe(OH) 4

−
. The electrode reaction produces abundant H + , which can prevent the deactivation of nZVI and enhance the dechlorination performance (eqn (11)).

R-Cl
There are also reports on the enhancement of nitrate removal and nitrogen selectivity by electrochemical method with magnetically immobilized nZVI anode on RuO 2 -IrO 2 /Ti plate, which possesses with ammonia-oxidizing function. 132The electrochemical method showed a nitrate removal efficiency of 94.6% and nitrogen selectivity of 72.8% at pH 3.0, and nitrate removal efficiency of 90.2% and nitrogen selectivity of 70.6% near a neutral medium (pH = 6).Wang et al. adopted tubular nitride carbon encapsulated nZVI as an electrocatalyst for the regulating  of multi-electron transfer reaction, which exhibited superior nitrate removal efficiency (92%) and N 2 selectivity (97%) in the pH range of 5-11 and recyclability (Fig. 18). 133The superior performance is attributed to the selectivity adsorbing of nitrate on the porous and hydrophilic nitride carbon layer, and the subsequent efficient cleavage of N-O bond by gaining electrons from nZVI and activated H.In addition, the generated intermediate NH 4 + can be oxidized to N 2 by HClO from the anode, thus leading to the high promotion of the production of N 2 (Fig. 18).

−
) is a disinfection by-product originating from the ozonation or chlorination of Br − in drinking water sources. 134 within 90 min. 134In the accomplishment of BrO 3 − reduction, direct reduction by nZVI, electrocatalytic reduction by nZVI, and electrocatalytic reduction by activated carbon ber account for 31.8%,41.9%, and 26.3%, respectively (Fig. 19).Recycle tests indicated this method has excellent performance during successive reduction.The reduction favors low dissolved oxygen concentration and pH, and high current intensity (Fig. 19).

Concepts for the future
Future research employing this innovative remedial technology may focus on the potential scale-up application at a contaminated site and the decontamination efficiency, or the reduction degradation of recalcitrant contaminants with the aid of a catalyst.The factors that affect the contaminant removal efficiency and energy consumption, in terms of corrosion voltage, corrosion current density, charge-transfer resistance, and the modication of nZVI on electrode, also deserves to be explicated.Furthermore, the strategy that realizing a circulation of formation, remediation, and regeneration of high reactivity nZVI in electrolysis-assisted application is also worth to be attempted.

Bacterium-assisted reduction by nZVI
Currently, the coupling treatment method of nZVI with microorganisms is a research hotspot in the eld of organic degradation, nitrate reduction, and metal ion removal. 135 Ma et al. employed a coupling system of nZVI and microorganisms to degrade polybrominated diphenyl ethers, and achieved complete debromination of BDE-209 within 30 days (Fig. 20). 137The addition of nZVI in sediment microbial fuel can reduce the oxidation-reduction potential of the system and thus enrich dechlorination microbial communities benecial for the reductive degradation of PCBs. 1380][141][142][143] The major removal improvement mechanisms involve chemical reduction by nZVI and dehalogenation bacterium-mediated microbial dissimilatory iron reduction (Longilinea and Desulfofustis, Dechloromonas sp., Shewanella putrefaciens CN32, Terrimonas, Lysobacter, Acidovorax, Dehalococcoides mccartyi, and Burkholderia ambifaria strain L3). 138,139,141,142,144,145Xu et al. proposed a removal mechanism of organic halides as: (i) organohalide-respiring bacteria utilize H 2 generating from nZVI corrosion to enhance dechlorination in the short term; (ii) the adsorption of nZVI materials and the promotion of dechlorination by attached biolm in long-term. 146Li et al. suggested that the addition of nZVI can signicantly increase the dehydrogenase activity of indigenous microorganisms in soil, and thus promote the removal of organochlorine pesticides. 147he addition of nZVI in anaerobic/anoxic/aerobic membrane bio-reactor can provide electrons for the denitrication, thus enhancing the reduction of nitrate. 148Similarly, Zhang et al. adopted biochar-supported nZVI in the cathodic removal of nitrated in a bioelectrochemical system to enhance the reduction of nitrate to NH 4 + . 149While the environmental-friendly reductant of nitrate is N 2 .It is worth mentioning that Zhou et al. used tea polyphenols to realize an excellent distribution and anti-oxidization of nZVI, achieving high conversion and selectivity in the transformation of nitrate to N 2 with the assistance of microorganisms (Fig. 20). 96,136ang et al. developed a novel porous phosphate-solubilizing bacteria beads loaded with biochar/nZVI in the enhancement of the passivation of lead in soil. 150nZVI makes the soil environment to be a relatively reduced state and thus promotes the release of soluble phosphate from soil, which can further form insoluble precipitates with Pd(II), transforming Pb(II) to a stable fraction alone in company with the reduction of Pb(II) by nZVI.
As CO 2 is one of the major greenhouse gases causing global climate change, its reduction has become a key global issue.nZVI is found to have the potential for biomethanation of CO 2 under anaerobic conditions in the bioreactor (Fig. 21), which relies on the formation of H 2 for hydrogenotrophic methanogenesis by nZVI (eqn ( 12)-( 14)). 151When nZVI is continuously added, 11.52% of the dissolved input CO 2 is transformed into CH 4 .While, the economic evaluation of this operation remains to be investigated, for the reason that the production of nZVI also required NaBH 4 , which is actually a good source of hydrogen.collaborative remediation methods involving multiple functional microorganisms with dehalogenase genes achieved by genetic engineering, and consider the inuence of eld environmental factors on the removal efficiency.Furthermore, the evolution mechanism of added nZVI in microbial-assist remediation warrants further exploration.

Conclusions
nZVI possesses high reactivity towards various contaminants and is attracting increasing attention.In the pursuit of superior and sustained reductivity of nZVI during application, many research has been conducted on the development of nZVI technology.Unfortunately, maintaining the high reactivity of well-dispersed nZVI and avoiding their aggregation during application is challenging, which is not only ascribed to the aggregation caused by magnetic attraction forces and strong van der Waal among nZVI particles, but also owing to nZVI the oxidization of nZVI by dissolved oxygen and water even in an anoxic condition.Consequently, nZVI is always encapsulated or supported in/onto porous materials to surmount these shortages and to obtain better reactivity and reusability, which incidentally endows nZVI materials with the functionality of adsorption towards pollutants.Additionally, nZVI tends to be deactivated during reduction via the precipitation of Fe ions, which conversely promotes nZVI materials with a novel functionality in the removal or recovery of pollutants via coprecipitation.This mini-review surveys the recent advances of various environmental remediation approaches by nZVI based on its reductivity, such as direct reduction, adsorption-reduction, reduction-precipitation, adsorption-reduction-precipitation, electrolysis-assisted reduction, and bacterium-assisted reduction towards organic halides, nitrate, metal ions, radioactive heavy metals ions, etc.However, there are still some challenges in the employing of nZVI in environmental remediation based on their reductivity: (1) nZVI is susceptible to deactivation during reduction, which is mainly caused by the formation of Fe-containing deactivation layer via oxidation or/and the precipitation of Fe ions, leading to a low utilization efficiency of Fe 0 , poor performance in recycling, and an unsustainable reduction performance during application.
(2) The current reports on the application of nZVI based on their reductivity are usually in acidic conditions, thus lacking approaches that are specically designed for the application in alkaline conditions.Meanwhile, there is also a lack of nZVI materials with higher reductivity for recalcitrant polyhalogenated aromatic hydrocarbons, which are widely distributed in waste circuit board dismantling materials and ameretardant materials.
(3) It has been veried that the dehalogenation by various noble metal doped nZVI involves both a hydrogen atom transfer mechanism and an electron transfer mechanism.While, which of these two mechanisms has more advantage on dehalogenation efficiency and the maintenance of the reactivity of nZVI during application, which of these two mechanisms is more applicable for specic contaminants, and which of these two mechanisms can realize a higher utilization efficiency of nZVI, are all remained to be investigated.
In closing, to address these issues and pave the way for the practical application of nZVI, future research may focus on: (1) Design and implement effective and environmentfriendly protocols for the comprehensive utilization of nZVI.For instance, developing techniques for the reconguration and re-activation of deactivated nZVI in environmental remediation, and developing consecutive and synergistic remediation strategies for various contaminants.
(2) Exploring highly adaptable nZVI materials for the reduction applications under complex actual environmental conditions, especially applicable in alkaline conditions and multiple co-existing interferences.
(3) The ingredients in actual polluted bodies can be very complex and may possess various distinct hydrophilic and hydrophobic characteristics, which poses high requirements for nZVI materials on their functionalities and surface characteristics.Thus, regulating the surface chemical and electrochemical properties are crucial for designing nZVI materials applicating in remediation via adsorption-reduction and adsorption-reduction-precipitation.
(4) Integrating more functionality into nZVI materials and thus realize more novel applications, such as detoxication and enrichment of microplastics and various contaminants adsorbed on.Meanwhile, efficient remediation of polluted soils with metals/metalloids and organic halides is a very tricky issue and has attracted increasing attention, which calls for the development of a new remediation strategy that combines physical, chemical, and biological remediation techniques together in nZVI materials.
(5) In an effort to obtain a thorough remediation of organic contaminants, reduction alone may be not enough.As dissolving Fe(II) generated during the reduction process can yet play a vital role in an extra Fenton oxidation process for the complete degradation of contaminant, future research may focus on the strategies of the combination of reduction and oxidation.
Yang et al. investigated the effect of nZVI anaerobic corrosion on the reductive dechlorination of TCE with various groundwater geochemical constituents (Na + , Ca 2+ ,

Fig. 1 3 −,
Fig. 1 Scheme of three dechlorination processes of TCE caused by diverse anaerobic corrosion mechanisms under the influence of Ca 2+ -HCO 3 − , Ca 2+ -SO 4 2− and Na + -NO 3 − within 8 days (8d), the insert pie charts (where the arrows point) are the proportional distributions of chlorine species during TCE reduction by nZVI, 29 copyright 2021 Elsevier.

Fig. 2 (
Fig. 2 (A) Schematic representation of the surface-mediated catalytic degradation of dechlorinated compounds by nZVI/Pd and nZVI/Ni; (B) comparison of various bimetallic nZVI on dechlorination performance, 28 copyright 2022 Royal Society of Chemistry.

Fig. 5
Fig. 5 Schematic representation of the adsorption and reductive dechlorination of TCB by modified biochar (with high aromaticity and hydrophobicity) supported nZVI/Pd; 39 SBC is NaOH-modified biochar and FBC is HF-modified biochar, copyright 2020 Elsevier.

Fig. 7
Fig. 7 Conceptual diagram for the mechanism of dechlorination of adsorbed PCB1 with different adsorption states regulated by three typical black carbon obtained at different pyrolysis temperatures, 42 copyright 2021 Elsevier.
Wang et al. conducted experiments to investigate the relative signicance of e-transfer and H-transfer in Cu, Ni, Pd, Ag, Pt, and Au doped nZVI in debromination of BDE-47, which primarily revealed that the debromination of BDE-47 by Fe/Ni, Fe/Pd and Fe/Pt follows H-transfer dominant mechanism, the debromination of BDE-47 by Fe/Ag follows e-transfer dominant mechanism, whereas e-transfer and H-transfer mechanisms may be equally involved in the debromination of BDE-47 by Fe/Cu and Fe/Au (Fig.

Fig. 10 (
Fig. 10 (A) Schematic mechanisms and pathways of debromination of polybrominated diphenyl ethers by Ni/Pd/Pt/Cu/Au/Ag doped nZVI systems; (B) possible debromination pathways of BDE-47 in various bimetallic systems; (C) debromination of BDE-47 in various bimetallic systems in a short period and (D) a long period, 46 copyright 2019 Elsevier.

Fig. 12
Fig.12The effect of Ag loading on the reduction of p-NP by nZVI, 52 copyright 2022 Elsevier.
15 (A) Illustration of the immobilization of Ni(II) by CnZVI; (B) removal efficiencies of Ni(II), Cr(VI), Cu(II), and Hg(II) with nZVI and CnZVI; (C) illustration of the adsorption of HPO 4 2− on the surface of nZVI; (D) time-sequence void evolution within iron spheres during Ni(II) removal with nZVI; (E) illustration of the Ni(II) removal process in a single sphere of nZVI; (F) time-sequence void evolution within iron spheres during Ni(II) removal with CnZVI; (G) illustration of the Ni(II) removal process in a single sphere of CnZVI, 63 copyright 2021 Wiley.

Fig. 16 (
Fig. 16 (A) Mechanistic illustration for simultaneous removal of 4-CP and Cr(VI) by Pd/Fe@PPY; effects of initial (B) Cr(VI) and (C) 4-CP concentration on the removal efficiency, (D) recyclability of Pd/Fe@PPY in the removal of 4-CP and Cr(VI), (E) effects of initial solution pH on the removal efficiency, 126 copyright 2022 Elsevier.(F) Schematic diagram of the possible adsorption-reduction-precipitation mechanism for Cr(VI) and U(VI); (G-J) sorption isotherms of Cr(VI) and U(VI) fitted by the isotherms equations, 71 copyright 2021 Elsevier.

Fig. 17
Fig. 17(A) Schematic diagram of the electrolysis-assisted nZVI system for the removal of selenite; (B) schematic diagram for proposed mechanisms of the removal of selenite; Se(IV) removal kinetics by the E-nZVI system at different (C) applied voltages and (D) electrolyte concentration, 131 copyright 2021 Elsevier.
Fig. 17(A) Schematic diagram of the electrolysis-assisted nZVI system for the removal of selenite; (B) schematic diagram for proposed mechanisms of the removal of selenite; Se(IV) removal kinetics by the E-nZVI system at different (C) applied voltages and (D) electrolyte concentration, 131 copyright 2021 Elsevier.
Yao et al. used nZVI immobilized activated carbon ber electrode for the electrocatalytic reduction of BrO 3 − , achieving a reduction efficiency of 94.2% of BrO 3 − (1.22 mM)

Fig. 18 (
Fig.18 (A) Mechanisms of catalytic denitrification on the nZVI@NC; the effect of (B) pH and (C) cycle times on nitrate removal and N 2 selectivity with nZVI@NC, 133 copyright 2021 Elsevier.

Fig. 20
Fig. 20 Schematic diagram of the highly efficient and selective transformation of nitrate to N 2 with the assistance of microorganisms and TP-NZVI/PE, 136 copyright 2020 Elsevier.
Fan et al. fabricated hydrotalcite-supported nZVI for the removal of methylene blue (MB). 50MB is rstly reduced to colorless leuco-MB, which is simultaneously attached to the surface of nZVI and nally removed by complexation precipitation.